Conventional CT (Computed Tomography) imaging systems measure the X-ray attenuation and provide limited contrast for medical imaging. Most clinical applications use contrast agents to enhance the contrast. However, it would be desired to extend the information contents of CT imaging systems.
In k-edge imaging a tuneable, monochromatic source is used for detection of specific atoms by measuring the attenuation at two or more energies, generally before and behind the k-edge, which is, for instance, described in H. Elleaune, A. M. Charvet, S. Corde, F. Esteve and J. F. Le Bas, “Performance of computed tomography for contrast agent concentration measurements with monochromatic X-ray beams: comparison of k-edge versus temporal subtraction”, Phys. Med. Biol. 47 (2002), 3369-3385. However, monochromatic sources are generally not suitable for clinical applications since they either have power levels far away from the required power for medical imaging or since they use synchrotron radiation of high energy accelerators.
WO 2007/034356 A2 discloses a CT imaging system using a conventional polychromatic X-ray source and an energy-resolving X-ray detector. With proper processing of the acquired data it is possible to reconstruct t least three images with a substance component (e.g. contrast agent component), a photo-effect component excluding the substance component and a Compton scatter component excluding the substance component. The X-ray detector provides a number of energy-resolved detection signals with spectral sensitivity for different energy bins, an energy bin being a section of the complete energy range in which said detection signal is available and of interest. The scanned object is then modelled as a combination of the photo-electric effect with a first spectrum, the Compton effect with a second spectrum and the substance with a k-edge in the interesting energy range with a third spectrum. The density length product for each of the components in each detection signal is modelled as a discrete linear system which is solved to obtain at least the k-edge components of said substance. From the k-edge components of said substance obtained for different detector positions a k-edge image of the substance can then be reconstructed with a conventional reconstruction method.
Spectral CT holds the potential to revolutionize CT imaging. Especially k-edge imaging will enable selective and quantitative imaging of targeted contrast materials. The major roadblock towards spectral CT, however, is the availability of detectors with very high count-rate capabilities.